Field of the Invention
Described herein are devices and methods relating to optical elements of surface mount devices, such as light emitting diode (LED) chips and components and more specifically, to primary optics and packages for surface mount devices and methods of manufacturing such optics and packages.
Description of the Related Art
Surface mount devices, such as LED-based light emitting devices, are increasingly being used in lighting/illumination applications. Semiconductor LEDs are widely known solid-state lighting elements that are capable of generating light upon application of voltage thereto. LEDs generally include a diode region having first and second opposing surfaces, and including therein an n-type layer, a p-type layer and a p-n junction. An anode contact ohmically contacts the p-type layer and a cathode contact ohmically contacts the n-type layer. In some cases, the diode region may be epitaxially formed on a substrate, such as a sapphire, silicon, silicon carbide, gallium arsenide, gallium nitride, etc., or growth substrate, but the completed device may have the substrate removed. The diode region may be fabricated, for example, from silicon carbide, gallium nitride, gallium phosphide, aluminum nitride and/or gallium arsenide-based materials and/or from organic semiconductor-based materials. In other configurations, it may be possible for the device to never include a substrate, such as if grown or processed on a virtual wafer.
Submounts are generally used in LED devices to interpose an LED chip and a printed circuit board. The submount may change the contact configuration of the LED chip to be compatible with the pads of the printed circuit board. The submount may also be used to support a phosphor layer or an encapsulating dome that surrounds the LED chip. The submount may also provide other functionality. Thus, a submount may include a receiving element onto which an LED chip is mounted using conventional die-attach techniques, to interface the LED chip and a printed circuit board. A submount generally has a thickness of at least 100 μm, and in some embodiments at least 150 μm, and in other embodiments at least 200 μm, and generally includes traces (such as on ceramic panels) and/or leads (such as in a Plastic Leaded Chip Carrier (PLCC) package).
The color or wavelength emitted by an LED is largely dependent on the properties of the material from which it is generated, such as the bandgap of the active region. It is often desirable to incorporate phosphors into a LED to tailor the emission spectrum by converting all or a portion of the light from the LED before it is emitted as it passes through.
The application of a conversion layer to an LED chip is typically done at the package level after the LEDs have already been singulated and subsequently bonded to an electronic element, such as a PCB. However, applying a conversion material at the package level rather than the wafer level is a less efficient manufacturing process, as it is much easier and cost effective to coat multiple LED chips simultaneously at the wafer level. Therefore, it is desirable to process steps at the wafer level or using a virtual wafer. It is also desirable to create devices which can forgo the use of an additional substrate or submount.
LED packages typically have some type of encapsulant surrounding the LED chip to enhance light extraction or beam shaping from the chip and protect the chip and related contacts structure (e.g. wire bonds) from exposure to physical damage or environmental conditions which could lead to corrosion or degradation. The lens can have a hemispherical shape and can be mounted to the package by the encapsulant.
LEDs typically emit light primarily into a hemispherical lens or in a hemispherical emission pattern. This confined emission, typically in the form of a Lambertian profile, is not generally suitable for many applications such as those requiring beam shaping; for example, collimated beam profiles, dispersed beam profiles, or specialized beam profiles.
For current state-of-the-art LED packages intended for lighting applications, a common configuration is the ‘surface mount’ package which incorporates one or more LED chips onto a planar substrate. One or more primary optical elements are then applied to this substrate, typically by a molding process. Present surface-mount LED package technology typically utilizes either a separate glass lens or a molded silicone lens. For surface mount packages, which typically require high temperature (200-300° C.) solder reflow processing to attach the LED package to its final fixture, the possible lens materials typically include silicones and glasses. These lenses are piecepart molded using known processes, such as injection molding, and are then affixed to the LED package.
By nature, the primary optical elements typically surround or encapsulate one or more LED chips and any associated electrical contacts. The preferred geometry for the primary optical element 10 has been a predominantly hemispherical shape, the manufacturing of which as shown in FIGS. 1A-1C. This shape has two primary benefits: (1) if large enough relative to the LED source, most of the light emitted by the LED is incident on the optic surface with a path that is nearly parallel to the surface normal (since the optic is typically surrounded by air and has an index of refraction higher than air, this minimizes the possibility of total internal reflection and hence efficiency loss), and (2) hemispherical shapes are readily fabricated onto planar surfaces by conventional molding processes. Furthermore, these hemispherical optics may suffer from output losses caused by total internal reflection (“TIR”) and the extraction efficiency of these domes remains low.
While the hemispherical optic 10 geometry is desirable with respect to efficiency and ease of fabrication, this geometry does little to modify the initial optical output beam profile from the LED source, outputting light in a Lambertian profile. In order to achieve light beam profile collimation, dispersion, or shaping, it is generally necessary to utilize a more complex optical geometry. However, many such geometries are not readily fabricated by molding processes. Specifically, since the mold cavity must be removed from the substrate surface following curing of the molded optic, it is not generally possible to mold parts which have ‘overhangs’, are tapered, or are narrower at the base, near the substrate, than at the top, as the substrate determines the orientation of the device within the mold.
As illustrated in FIGS. 1A-1C, in traditional molding, a mold 108 is applied to a planar substrate 100 with associated LED chips 102. The cavities in the mold are filled with a suitable encapsulating/optical material such as silicone or epoxy. The encapsulant is then at least partially cured, and the mold removed, leaving behind encapsulant on the surface in the form of a primary optical element. In order to remove the mold from the primary optic after at least partial curing, it is necessary that there are no regions of ‘overhang’ which would prohibit mold removal. This limitation in particular can inhibit the molding of many collimating-type optics. While there are molding techniques which can allow overhang geometries, they typically involve complex molds with moving parts which are not suitable for batch fabrication of many molded elements in an array on one surface. Additionally, the inclusion of a substrate interferes with these processes. Undercuts on optics require mold pieces to pull out sideways. These mold types are called side actions. Furthermore, traditional molds must be used; a more efficient dispensing process without a mold cannot be utilized to create complex primary optics for a plurality of LEDs on a surface because molds are currently required to provide complex shaping.
As a result of these limitations, beam shaping is typically achieved through the use of ‘secondary’ optics. Such secondary optics generally increase the overall cost and reduce efficiency. Further, the shape of the secondary optic can be limited by the size and geometry of the primary LED optic or lens—this can further reduce efficiency and limit the potential for beam shaping in some applications, particularly those involving collimation of the LED light, where it is helpful to bring the optical element as close to the light source (LED chip or chips) as possible. The use of secondary optics can result in lighting solutions which are bulky, require additional design work and alignment, optical loss, and additional costs. Therefore, it is desirable to be able to create primary optics with overhangs or undercuts in a more efficient manner.